1. Field of the Invention
The invention relates generally to an apparatus and a concomitant method for retaining a workpiece within a semiconductor wafer processing system and, more specifically, to an improved electrostatic chuck that provides zones of varying chucking force to reduce heat transfer gas leakage, and improve heat transfer gas layer uniformity beneath the workpiece.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. Since the materials and processes used to process a wafer are extremely temperature sensitive, temperature control is an important aspect of wafer processing. Should the materials be exposed to excessive temperature fluctuations resulting from poor heat transfer during processing, performance of the wafer process may be compromised resulting in wafer damage. As such, the pedestal may form a chucking electrode as well as a heat sink or heater as used in etching, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applications.
To optimally transfer heat between the wafer and pedestal, a very large, nearly uniform electrostatic force is used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the chuck. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that reduce contact area and thus reduce optimal heat transfer. To improve heat transfer during processing, an inert heat transfer gas is pumped into the volume formed by the interstitial spaces between the support surface of the chuck and the wafer. This gas acts as a thermal heat transfer medium from the wafer to the chuck that has better heat transfer characteristics than the vacuum it replaces.
Since the distribution of heat transfer gas to the interstitial spaces is osmotic and the interstitial spaces may not be interconnected, some spaces do not receive any heat transfer gas. This condition leads to a non-uniform temperature profile across the backside of the wafer during processing and results in wafer damage. The uniformity of the chucking force serves to compound the problem as the wafer is held in place evenly across the support surface. Since effective and uniform heat transfer from the wafer is an important aspect of the manufacturing process, maximizing wafer exposure to the heat transfer gas contributes to the greatest heat transfer rate.
In addition to maximizing wafer exposure to the heat transfer gas, minimizing leakage of the heat transfer gas is also a wafer processing concern. Traditionally, the electrostatic chuck or the pedestal which supports the chuck contains an outer diameter raised rim. The rim is approximately the same diameter as that of the semiconductor wafer. As such, the wafer is supported at its outer edge by this rim. When an electrostatic clamping force is applied to the chuck, the wafer is pulled down to the support surface effectively creating a seal on the backside of the wafer at the rim. As discussed previously, a heat transfer gas into the process chamber is introduced to the backside of the wafer to improve heat conduction away from the wafer. Unfortunately, gas pressure drops significantly at the edge of clamped wafer because the volume of the interstitial spaces increases as the radius of the wafer increases. Additionally, a large amount of heat transfer gas leakage is known to occur at the rim. The clamping force pulls the wafer down while pressure from the heat transfer gas on the backside of the wafer pushes the wafer up causing the wafer to flex during processing. These opposing forces reduce the area of the wafer that is in contact with the rim. If the wafer is not seated properly on the support surface, a sudden reduction in chucking force occurs or a sudden increase in heat transfer gas pressure occurs due to an anomaly during wafer processing, substantial leakage of the heat transfer gas occurs at the reduced contact area.
Existing art in electrostatic chucks is limited by the uniform chucking force that chucks the wafer to the support surface. The uniform chucking force dictates that the wafer will be subject to the same downward force across the entire support surface regardless of characteristic weaknesses (i.e., reduced contact at the outer diameter rim and non-uniformity of heat transfer gas diffusion in some interstitial spaces) of the chuck. Two examples of such apparatus are disclosed in U.S. Pat. Nos. 4,384,918 issued May 24, 1983 to Abe and 5,452,177 issued Sep. 19, 1995 to Frutinger.
Therefore, there is a need in the art for an improved apparatus for retaining a wafer that modulates electrostatic chucking force across the support surface. Greater force is applied where it is needed most and reduced where uniform heat transfer gas diffusion would otherwise be inhibited. As such, temperature uniformity across the bottom surface of the wafer is attained.